Long-Term Cap. Gains - Based on Before or After Standard Deduction

Happy Friday!

I'm looking to understand the long-term capital gains (LTCG) tax as it relates to the thresholds that determine tax paid on gains. In years past, I've made more money, but went back to school and made less this year while in school. So my circumstances changed this year that might allow me to harvest some long-term capital gains at 0%. (Ex: sell some stocks and buy back since wash sale doesn't apply to capital gains). But I'm not sure how the up to $40,400 filing single for 2021 threshold is determined.

--> My question: Is the taxable income figure that governs LTCG calculated before or after taking the standard deduction? <--

Here's what I mean by providing some example values for 2021:

Earnings: ~$47,000, 401k Contributions: ~$2,000, American Opp. Credit: $2,500

This means my AGI will be roughly ~$42,500 before taking the standard deduction. For the purposes of determining LTCG, which of these two scenarios is correct?

Option A: AGI of $42,500 --> Over the $40,400 threshold. Any LTCG will be taxed at 15% (I certainly don't have anywhere near the next level at $400,000 hahaha)

Option B: $29,950 ($42,500 minus single Std. Ded. for 2021 of $12,550) --> Under the $40,400 threshold. Up to $10,450 of LTCG will taxed at 0% aka not taxed. ($40,400 minus $29,950 = $10,450 of potential LTCG untaxed)

Thank you!